Half life

```					HALF LIFE
will eventually lose their
• We can measure how quickly a
measuring the half-life.
• This is the average time taken
for half the atoms to decay
HALF-LIFE
• Plot the following data.
COUNT
RATE
(bequerels)
95      73   55   42    32    23   18
Time           0    10   20   30    40    50   60
(secs)

Count rate

Time
• Complete the following

• The time at 90 counts is …………….

• The time at 45 counts is …………….

• 45 is half of 90 so the time it takes for the
counts to drop by half is ________

• Pick two more pairs of counts where one
is half the other and work out the half-life.
HALF-LIFE
R
a
d
i
For this substance
o
a                  Half life – the time (37
c                  hours) for the
i                  half ( 16 to 8)
v
i
t
y

Time (hours)
ANOTHER WAY TO CALCULATE HALF
LIFE

• Instead of drawing the graph we can
calculate the half life from the data
given.
• EXAMPLE
• A radioactive isotope has a count rate
of 400 becquerels. 12 days later it is
50. What is the half life?
After 1           After 2            After 3

half life         half lives         half lives
• 400               200                100                50

• 3 halflives have passed for the counts
to fall from 400 to 50
• This took 12 days.
• So 12 (days) / 3 (halflives) = 4 days.
• The half life is 4 days
substance will have it’s
own half life.